Dark Matter Gets Its Day

A new cosmological celebration, coinciding with Halloween, seeks to shed light on one of the great mysteries of the universe: dark matter.

Photograph by Irina Dmitrienko / Alamy

Not long ago, the actor Tilda Swinton—cosmic muse to cinéastes, fashion
designers, and physicists—took on another shape-shifting role as the
voice of a new a planetarium film, “Phantom of the Universe: The Hunt
for Dark Matter.” “As we look out
into the night sky, we are both dazzled and comforted by the patches of
light we find there,” her narration begins. In time, Swinton continues,
astronomers started to suspect that there was something more out there
than these brilliant moons, stars, and galaxies—“something hiding in
the dark spaces.” The film premièred in Mexico City, on Sunday, and
today has special showings worldwide in celebration of this, the
inaugural Dark Matter Day.

Everything that humans have seen up until now exists in the 4.9 per
cent of the universe that interacts with light. The rest is hidden from view. Most of it, physicists believe—68.3 per cent—is dark energy, an enigmatic force that
drives the accelerating expansion of the cosmos. The rest—26.8 per
cent—consists of dark matter, a ghostly goo that is thought to hold the
cosmos together. This is why the Interactions Collaboration, a global
consortium of particle-physics laboratories, has reimagined Halloween as
Dark Matter Day. “Dark matter seems to ‘hide’ in plain sight and doesn’t
play by the known rules of physics,” a promotional
F.A.Q. explains.
“It’s like a costumed trick-or-treater who rings the doorbell and then
dashes away, and scientists are trying to unmask it!”

Dark matter was first theorized, in the nineteen-thirties, by Caltech’s
Fritz Zwicky, who reputedly referred to his colleagues at the Mount
Wilson Observatory, in Los Angeles, as “spherical bastards,” since he
found them equally disagreeable from all sides. (Costume idea!) Forty
years later, Vera Rubin, of the Carnegie Institution for Science, in
Washington, D.C., confirmed Zwicky’s theory. Studying the rotation of
galaxies, Rubin and her collaborators observed that, given the galaxies’
spiralling speeds, and given their visible mass, these stable structures
should in fact be flying apart. This amounted to circumstantial evidence
that an invisible incarnation of matter—a halo, as it’s occasionally
called—kept them whole.

Now thousands of physicists have joined the hunt. But looking for the
subatomic source of dark matter—the leading candidate is known as the
WIMP, for weakly interacting massive particle—has proved an expensive
and frustrating, if occasionally edifying, odyssey. At the European
Organization for Nuclear Research (CERN), in Switzerland, where Dark
Matter Day will be celebrated with Dark Matter Cake—baked with the
cosmically correct proportions of white-chocolate chips (visible
matter), dark-chocolate chips (dark matter), and beetroot (dark
energy)—the universe’s mystery ingredient is “definitely in the
spotlight now,” Oliver Buchmueller, a senior physicist at Imperial
College London, told me. Now that the Higgs
boson is well accounted for, dark matter has become one of the Large Hadron
Collider’s main targets. The favored model for predicting dark matter
has long been supersymmetry. As Swinton explains in the film, “According to this
theory, for every known particle, like an electron or quark, there’s a
corresponding superparticle with a much greater mass.” But since none of
these partners have shown themselves at the L.H.C., researchers are now
testing more generic scenarios. “One hypothesis is that the Higgs could
be a portal connecting us to the dark world,” Buchmueller said. “We know
that the Higgs boson gives mass to all our fundamental particles. But,
instead of just decaying to these particles we know in the visible
world, the Higgs might also decay to the dark-matter particles.” So far,
though, no dice.

The same is true at SNOLAB, in Sudbury, Ontario, a facility buried more
than a mile underground, in an active nickel-and-copper mine. Here the
goal is to actually observe dark-matter particles as they pass through
the planet; the overlying rock filters out the noisy cosmic rays that
would otherwise smother the signal. On a visit in August of last year, I
joined a group of miners aboard the 8:05 A.M. cage, travelling downward
at twenty-five miles per hour. (Thankfully, I didn’t faint; enough
SNOLAB visitors do that the miners call them SNOflakes.) I was there to
see the DEAP-3600 liquid-argon detector, which, after six years in the
making, was just about to become operational. The ultra-pure argon,
cooled to minus 297 degrees Fahrenheit, is meant to serve as a conduit
of sorts, lighting up in the presence of dark matter. Almost a year
later, in July, 2017, the DEAP team published its first results: “No
candidate particles are observed.” As Richard Gaitskell, the spokesman
for the equally unsuccessful Large Underground Xenon detector, in the
Black Hills of South Dakota, told me, “So far we’ve always gotten a
negative result. Which means we only know what dark matter isn’t.” The
upshot, he said, is that “there are basically thousands of models of
particle physics lying bloodied in the gutter.”

Along the way, however, there have been consolation prizes. In
September, 2009, M.I.T.’s Tracy Slatyer and her colleagues analyzed new
data from the Fermi Gamma-Ray Space Telescope and spotted a fuzzy blob
of gamma rays extending far above and below the core of the Milky Way
galaxy. Could this be a relic of dark matter, they wondered? Alas, it
turned out to be something else—“Just something we hadn’t dreamed of
yet,” Slatyer said: a figure-eight-shaped pair of bubbles, likely an
eruption from a black hole five million times as massive as our sun.

In examining these so-called Fermi bubbles, Slatyer and another team
noticed something more: an
excess of gamma rays emanating from the galactic center. “We believe
that dark matter piles up in the center of galaxies, because it’s pulled
there by gravity,” she told me. That makes the galactic center a good
place to look, though also a frightening place, she noted, because it’s
populated by so many violent and high-energy astrophysical phenomena.
The gamma-ray excess could come from dark matter, or it could come from
a population of rare millisecond pulsars—city-sized neutron stars
spinning around at a rate of a thousand times per second. Slatyer is
ninety-five per cent confident that this is another false alarm. (Her
more optimistic colleague gives dark matter a fifty-fifty chance.) But,
Slatyer said, “the best thing about these false alarms in astrophysical
data is that even if they turn out not to be dark matter, they often
tell you about something very interesting. You get a discovery either
way.”

Perhaps the most pessimistic proposition involves the recent revival of
a radical theory from the nineteen-eighties known as MOND, or modified
Newtonian dynamics, which hypothesizes that there is no dark matter—none
at all. Rather, the galactic conundrum is solved by a shift in our
understanding of gravity. “When I was a kid, I would wake up one night
out of every thirty and think, Oh, my God! It’s probably MOND!” Nima
Arkani-Hamed, of the Institute for Advanced Study, in Princeton, told
me. “And the other twenty-nine nights, I would be happy that it was
probably dark matter. Then I became a scientist, and now it’s once a
year that I’ll look up and be, like, Oh, my God. Maybe it’s MOND. But I
don’t think it is. It doesn’t smell right to me.”

But, then again, the worst-case scenario is that, in ten years, or a
hundred, this spooky predicament remains a mystery. Sure, the joy is in
the hunt—and, as Swinton concludes in her narration, “Ultimately, it’s
the big questions that bring humankind together”—but to spend lifetimes
searching for something and not finding it would be, well,
astronomically frustrating. “The problem is, we have no idea what we are
looking for,” Hugh Lippincott, of the Fermi National Accelerator
Laboratory, outside Chicago, said. “And there is a not insignificant
chance, probably better than fifty per cent, that we are never going to
find it. That’s the scary part.”

Siobhan Roberts is the author of “Genius at Play: The Curious Mind of John Horton Conway.”